In the medical field, it is known to use light, especially with wavelengths located in the infrared region, in order to study in some depth the nature of biological tissue.
However, biological tissue is highly scattering at optical wavelengths, which tends to blur all information relating to its internal structure. Now, this structure influences the data and it may be necessary to assess this influence. For this, it is beneficial to study the dynamics of light propagation in biological tissue. To this end, it is known to send a very short light pulse to the medium to be studied and to carry out time resolved measurements of the scattered light. Time resolution provides valuable information concerning the structure of the medium.
The measurements may be carried out in reflectance or in transmittance.
The reflectance measurements are used especially in order to determine the oxygenation level of the blood hemoglobin.
The transmittance measurements are used especially for detecting cancerous tumors of the breast. The images obtained by time resolved transmittance measurement of the scattered light produce high contrast images making it possible for a tumor to be clearly detected therefrom. However, the spatial resolution of the images is relatively mediocre.
This limitation arises from the technology used for implementing the measurement. This is because the time resolution needs to be carried out over a very short time of about a few tens of picoseconds. Thus, such resolution requires highly sophisticated and expensive technologies.
In the known observation devices, the source producing the light pulse is adapted to provide a very brief pulse.
For example, in the device described in document U.S. Pat. No. 5,692,511, the light source used is a titanium-sapphire laser. The latter is very expensive. Furthermore, the detection is awkward to implement. The solution utilized in the device described in this document uses an avalanche photodiode connected to an electronic time gate in order to determine the number of photons transmitted during a particular time period. The avalanche photodiode has a relatively long time response which degrades the potential contrast of the time gate.
More generally, in devices for analyzing scattered light by time resolved measurement, it is common to use a source making it possible to produce brief pulses such as pulsed lasers, which are expensive.
Furthermore, means used for detecting the light present problems specific to each of them which do not allow satisfactory use of the device.
From these means of detecting the signal, it is possible to distinguish:                fast detectors with electronic processing of the signal coming from the detector, the relatively long response time of which considerably degrades the contrast of the time gate;        time amplitude converters in photon counting mode which require decreasing the signal in order to be placed in a suitable mode, which is not compatible with the short acquisition time;        optical gates with a nonlinear effect which require powerful lasers with low rates of about 10 Hz, which considerably limits the sensitivity;        slit scanning cameras which have very high time resolution (about 2 ps), but the dynamic range of which is not very high; and        ultrafast intensified cameras, the time resolution of which is not very high (about 80 ps) for a low repetition rate (about 1 kHz).        
Thus, currently available technologies do not make it possible to obtain good spatial resolution, especially with scattering tomography.